A man who raped his step-granddaughter over six years has been sentenced to 20 years in prison. The girl had two children as a result, according to court documents.

Suwon District Court said Thursday that the man, identified only as "A," 53, had raped the girl, "B," 17, since she was an elementary school student.

The attacks started when the girl moved into his house in 2011.

B is a granddaughter of the woman "A" lived with without official marriage registration. The woman took custody of the victim after her parents divorced.

The man was able to keep his attacks secret because he raped the girl when her grandmother was absent, according to court documents. He also told the girl: "I will kill you if you speak out about this."

The grandmother learned of the shocking story early this year and reported it to police.

"It's horrendous and shocking," the court said. "Even though he has no criminal history, a heavy punishment is inevitable."

The man claimed sex with the victim was "always consensual" and he did not know about the babies.